I think at least you have a good defense available- you believed in good faith that what you were doing was innocent. No "mens rea". A caretaker, as far as you know, IS the agent for the owner. What makes anyone an agent? It comes down to reasonable belief. Digging holes in open land is hardly "damage", and open-land trespass does require certain warnings, fences, etc, and I think you have to be told to "move on", first, before you can be charged with, what is essentially, "defiant trespass", which no one is claiming.
Go to
www.findlaw.com, look up the Texas Code, and pull up the definitions of the charges you are facing. Post them here. There may even be references within each statute, to other relevant law.